Superheterodyne frequency changer



A. l. F. SIMPSON ET AL SUPERHETERODYNE FREQUENCY CHANGER Filed Jan. 25, 1940 Aug. 13, 1940.

Patented Aug. 13, 1940 UNITED STATES PATENT GFFICE SUPERHETERODYNE FREQUENCY CHANGER Application January 25, 1940, Serial No. 315,484 In Great Britain January 20, 1939 6 Claims.

This invention relates generally to ultra-highfrequency carrier-signal receivers of the superheterodyne type and particularly to such receivers comprising frequency changers adapted to change the frequency of incoming signals whose frequency is, for instance, greater than 25 megacycles, and particularly of the order of 100 megacycles, by combining them with oscillations generated by a local oscillator whose frequency is relatively low, for instance, less than 25 megacycles.

Ultra-high-frequency carrier-signal receivers of the superheterodyne type include frequency changers which must be supplied with ultra-highfrequency local oscillations of stable frequency in order to procure an intermediate-frequency signal of suitably low and constant frequency. However, it is difficult to provide a stable local oscillator whose frequency is much greater than 30 megacycles; for above this limit crystal oscillators are diliicult to make and resonant circuits of capacitance and inductance are greatly affected by stray impedancesand temperature variations. Accordingly, it is usual, in frequency changers of the type specied, to use a local oscillator of comparatively low frequency, for example, of the order of l0 megacycles, and to multiply, for example, double, the frequency in one or more frequency-multiplying stages before the local oscillations are combined in the frequency changer with the incoming signals. Each of these frequency-multiplying stages usually comprises a thermionic tube, but it does not produce any amplification in the signal-translating channel of the receiver with which the frequency changer is associated. Accordingly, the intermediate-frequency signal output of the frequency-changing stage, wherein the local oscillations of multiplied frequency are combined with the incoming signals, are often undesirably weak.

It is the primary object of this invention, therefore, to provide an ultra-high-frequency carriersignal receiver of the superheterodyne type in which the intermediate-frequency signal has greater amplitudel than that provided by prior art arrangements using the same number of tubes and providing the same intermediate frequency.

In accordance with the invention, an ultrahigh-frequency signal receiver of the superheterodyne type, including a signal-translating channel, comprises a low-frequency local oscillator, frequency-multiplying means coupled to the oscillator, and a plurality of frequency changers coupled in cascade in the signal-translating channel, each having a conversion gain greater (Gl. Z50- 20) than unity and being effective to reduce the signal-carrier frequency. Means are provided for deriving from the frequency-multiplying means high-frequency input oscillations for each of the frequency changers, the input oscillations for one of the frequency changers being derived from another of the frequency changers through a selective means included in said last-mentioned means whereby an intermediate-frequency signal which has a carrier frequency greatly reduced from that of the received signal and which has a substan-y tially greater amplitude than that of the received signal is derived from the last of the frequency changers in said channel.

For a better understanding of the invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

Figs. l and 2 of the drawing are schematic circuit diagrams of carrier-signal'receivers of the superheterodyne type of known form and are used to explain the operation of the present invention, while Fig. 3 of the drawing is a circuit diagram, partially schematic, of a modification of the present invention incorporated in an ultra-high-frequency carrier-signal receiver of the superheterodyne type'.

The fact, well known per se, on which the present invention depends is that frequencychanging stages of known type, in which two oscillations to be combined are applied to control the same electron stream, are amplifiers, having usually about one-third of the voltage amplification that the same tube would have if it were used as a conventional amplifier with the same anode load. The proposal of the invention is, therefore, generally, to substitute a frequencychanging stage providing amplification along the signal-translating channel for one 0r more of conventional frequency-doubling stages yielding no such amplifie-ation.

These statements will be readily understood by those skilled in the art but may be amplified to explain the essential characteristics of the invention more formally. I-f a voltage ey cos wgt is applied to the input circuit of an amplifying tube, the output voltage across a load of impedance Za at the angular frequency wg in the anode circuit of the tube is ea cos wgt, where ea=egyaZa and ya, the mutual conductance, depends on the construction of the tube. The product gaZa is the amplification. If a voltage eg cos wgt" is applied to the input circuit of a frequency changer to Cal which is also supplied local oscillations en cos wot, then, so long as the amplitude eo exceeds a certain lower limit, the signal output of angular frequency log-wo] across a load of impedance Zf at that frequency in the anode circuit of the tube is e1 cos (wg-wo) where ef=eggfZf and gf is the conversion conductance of the tube. The product UfZf is then the amplification along the signaltranslating channel of the applied signal. The two facts vital to the invention, therefore, are:

(1) The voltage er is independent of e0 and wo so long as e0 is always above some lower limit, and perhaps below some other higher limit, so that, if e0 is above that lower limit, as is always to be assumed, increase of e0 does not increase cf or the amplification along the signal-translating channel of the applied signal.

(2) If the tube of the frequency changer is the same as that of the amplier, gf is approximately grt/3.

Accordingly, if the comparison between the amplifier and the frequency changer is made, as it should be, on the assumption of equal output impedance, the amplification of the frequency changer will be one-third that of the amplifier and can conveniently be made greater than unity. In what follows, amplification, unless qualified, will always mean an amplification greater than unity.

This idea of the resent invention will now be further explained with reference to Figs. 1 and 2 of the drawing. In Fig. 1 of the drawing there is represented an ultra-high-frequency carriersignal receiver of the superheterodyne type including an antenna-ground system 8 and 9, to which is coupled, in cascade, a radio-frequency amplifier Iii, a frequency changer 2B to provide an intermediate carrier-frequency signal, and a detector, modulation-frequency amplifier, and signal reproducer H. A source of local oscillations comprising local oscillator l2 and frequency doublers i, 2, 3, and i is coupled to frequency changer 2li. The operation of the circuit illustrated in Fig. l will be readily understood by those skilled in the art.

Fig. 2 is a schematic circuit diagram of an ultra-high-frequency carrier-signal receiver of the superlieterodyne type which is similar in some respects to the receiver of Fig. l and correspondunits or elements have the same reference symbols. The receiver of Fig. 2 differs from that of 1 mainly in that an additional frequency changer 2l has been added coupled in cascade with frequency changer 2Q, frequency doubler l being omitted and the oscillation voltage output fron-i frequency doubler 3 being applied to each of frequency changers 23 and 2 i. In order to explain the operation of the invention, a comparison will be made between the receiver of Fig. l and that of Fig. 2 and, in order to provide a suitabie comparison of the two receivers, it is assumed that signals of the frequencies indicated in the drawin T are translated or developed by the different units of the receivers.

In the receiver of Fig. 1 there is a single freq1 1encychanging stage 28 to which are applied (l) the incoming signals from radio-frequency amplier Eil and (2) the local oscillations from oscillator' i2 after their frequency has been multiplied 16 times by passage through the four frequency doublers i, 2, E, and 3. The intermediate frequency of the signal applied to unit H is, therefore, 24U-924:16 inegacycles.

1n the receiver of Fig. 2, there are only three frequency doublers i, 2, and 3, but there are two frequency-changing stages 20 and 2| to the first of which is applied (l) the incoming signals derived from unit i0; and (2) the local oscillations after their frequency has been multiplied eight times; while to the second of which is applied (l) the intermediate-frequency signal derived from the first frequency changer 2U; and (2) the local oscillations from oscillator I2 after their frequency has been multiplied eight times. The intermediate frequency applied to unit Il is, therefore, 2l0-112-112=16 megacycles,

ln the receiver of each of Figs. 1 and 2 there are ve doubling and frequency-changing stages cach of which includes at least one vacuum tube; but, whereas in the receiver of Fig. 1, there are four frequency doublers giving no amplification along the signal-translating channel of the receiver and one frequency changer which provides amplification in the signal-translating channel, in Fig. 2 there are only three frequency doublers, but there is a second frequency changer which also provides amplification in the signal-translating channel of the receiver. For input oscillations cf the same amplitude, therefore, the amplitude of the ultimate intelmediate-frequency signal which is applied to unit l l is, therefore, greater in the arrangement of Fig. 2 by the amplification provided by frequency changer 2l.

It is technically possible to extend this substitution; thus, frequency doubler 3 might be removed and replaced by a frequency changer in cascade with those shown in Fig. 2, all the frequency changers being fed with oscillations of a frequency of 56 megacycles from the output of the frequency doubler 2. However, in this case, in order to obtain the ultimate intermediate frequency of 16 megacycles, four frequency changers are required in the signal-translating channel of the receiver, since in each the frequency is reduced only by 56 niegacycles. Accordingly, least six tubes would then be required in place of the original five. It is true that the amplitude of the ultimate intermediate-frequency signal would be greater; but that could be achieved equally well, and probably more simply, by adding a sixth tube as a conventional amplifier in the signal-translating channel of the receiver. Again if the multiplier replaced by a frequency changer multiplied in a ratio greater than 2, it could not be replaced by a frequency changer without increasing the ultimate intermediate frequency. It appears, therefore, that, so long as the tubes are all either frequency multipliers or frequency changers, as indicated in Fig. l, it is desirable to replace only one multiplier by a freque-ncy changer and then only when the multiplier is a doubler. On the other hand, there is ne reason why any frequency multipliers that are not replaced should not multiply by a factor greater than 2.

In the arrangement shown in Fig. 2 the input circuits of the two frequency-changing stages 20 and 2i have a common part, namely, the output circuit of the frequency doubler 3, through which there may exist a feed-back coupling that may give rise to instability and distortion. This feature can be avoided by making use of the fact that the signal-frequency components supplied to the input circuit of a frequency-changing stage appear also in its output circuit, in addition tc the intermediate-frequency signal arising from their combination. Usually none but the combined or heterodyne-frequency components are utilized. Accordingly, the output of the frequency changer 20 of Fig. 2 contains an intermeydiate-frequency filter which selects the combined or heterodyne-frequency components and bypasses the uncombined frequency components from the signal-translating channel of the receiver.

A modification of the invention is, therefore, provided by breaking the connection between frequency doubler 3 and frequency changer 2| in Fig. 2 and replacing it by a connection between the two frequency changers 20 and 2| which permits oscillations of the frequency of those leavingfrequency doubler 3, as well as their heterodyne-frequency components, to reach the second frequency changer 2|. This arrangement is represented by the schematic circuit diagram of Fig. 3, in which circuit elements which are similar to those of Fig. 2 have identical reference symbols. Means are therefore provided for deriving from the frequency-multiplying means 3 high-frequency input oscillations of the same frequency for each of frequency changers 20 and 2|, this means including selective means 24 which is effective to pass only oscillations of the given frequency coupled between frequency changers 2911 and 2|. An advantage of the arrangement of Fig. is that if volume control, for example, automatic amplification control or A. V. C., is applied to the first frequency changer 20 to decrease the amplification of the translated signals, the amplitude of the oscillations applied therefrom to frequency changer 2| is also decreased. If the translated signals are reduced below the aforesaid lower limit for eo the signal output of frequency changer 2| is decreased by virtue of the decrease of et as well as by virtue of the decrease of eg, and the effect of the volume control is thus greater.

A further modification is possible by virtue of the presence of harmonic-frequency components of the local oscillations in the output of a frequency changer. Thus, in Fig. 3 frequency doubler 3 might be omitted, so that the local oscillations applied to frequency changer 20 are .of a frequency of 56 megacycles and their combination with the translated carrier signal is of a frequency of 184` megacycles. If filter 24 is then designed to transmit the third harmonic of the local oscillations applied to frequency changer 2S, namely, 168 megacycles, the intermediate-frequency signal developed by frequency changer 2| still has a frequency of 16 megacycles. In this modification of the invention, the amplitude of the local oscillations at the input circuit of frequency changer 2li must be of a value to produce a sufficiently strong third harmonic at its output.

Suitable frequency-selective means for filters 23 and 24 in Fig. 3 may readily be provided by those skilled in the art. Thus, the coupling between the two frequency-changingstages may comprise two' parallel-tuned circuits connected in series in the anode circuit of frequency changer 2li, one tuned to the heterodyne frequency at the output of frequency changer 20, for example, 128 megacycles, and the other to the input frequency to frequency changer 20 from frequency doubler 3, for example, 112 megacycles. Alternatively, the coupling between the two frequency-changing stages may comprise two coupled paralleltuned circuits, each tuned to a frequency between the heterodyne frequency of the output of frequency changer 20 and the frequency of the output of frequency doubler 3, so as to have minima of attenuation at the two frequencies. The signals of the two frequencies may be applied together to the grid of a frequency-changing tube or may be separated in known manner and applied to separate grids. f

In summary, the invention in one form can be generally stated as follows: A superheterodyne frequency changer of the type specified comprises a pair of frequency-changing stages A and B, each stage having an amplification or conversion gain greater than unity. A portion of the signal input to each of stages A and B consists of oscillations whose frequency JA or fa, respectively, is a multiple of that of the oscillations generated by a local oscillator which are thereafter multiplied by frequency-multiplying stages, of which one may be the frequency-changing stage A. The oscillations of frequency fB supplied to frequency changer B are supplied thereto through frequency-changer stage A. The other portion of the signal input to stage A is the incoming signal of frequency F, while the other part of the input to stage B is an oscillation of frequency F-fA derived from the output of stage A, so that an intermediate-frequency signal of frequency F-fA-JB may be derived from stage B. The 'arrangement of the invention is such that the derived intermediate-frequency signal has a greater amplitude than it would have if it were developed by a frequency changer derived from the frequency changer of the invention by the omission of stage A and by the addition of a frequency-multiplying stage for multiplying the frequency of the locally-generated oscillations, and having applied to the input of the frequencychanging stage B a frequency of fA-l-JB.

It will be understood that local oscillator |2 andthe selective circuits of unit may be tunable and that the tuning units may be aligned for unicontrol. In this case, due to the fact that the ultimate and constant intermediate frequency derived from frequency changer 2| is determined by the action of the two frequency changers 2| and 22, the rate of change of tuning of local oscillator l2 is only half that for conventional superheterodyne receivers comprising a single frequency changer.

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious. to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. An ultra-high-frequency carrier-signal receiver of the superheterodyne type including a signal-translating channel comprising, a lowfrequency local oscillator, frequency-multiplying means coupled to said oscillator, a plurality of frequency changers coupled in cascade in said channel each having a conversion gain greater than unity and being effective to reduce the signal-carrier frequency, and means for deriving from said frequency-,multiplying means highfrequency input oscillations of the same frequency for each of said frequency changers, said lastmentioned means comprising selective means coupled between said frequency changers which is effective to pass only oscillations of said frequency, whereby an intermediate-frequency signal which has a carrier frequency greatly reduced from that of the received signal and which has a substantially greater amplitude than that of the received signal is derived from the last of said frequency changers.

2. An ultra-high-frequency carrier-signal receiver of the superheterodyne type including a signal-translating channel comprising, a loW- frequency local oscillator, frequency-multiplying means coupled to the oscillator, a plurality of frequency changers coupled in cascade in said channel each having a conversion gain greater than unity and being effective to reduce the signal-carrier frequency, and means for deriving from said frequency-multiplying means highfrequency input oscillations of the same frequency for each of said frequency changers, said last-mentioned means including selective means for deriving input oscillations for the second of said frequency changers from the rst of said frequency changers, whereby an intermediatefrequency signal which has a carrier frequency Igreatly reduced from that of the received signal and Which has a substantially greater amplitude than that of the received signal is derived from the last of said frequency changers.

3. An ultra-high-frequency carrier-signal receiver of the superheterodyne type including a signal-translating channel comprising, a low-frequency local oscillator, frequency-multiplying l,means coupled to said oscillator, a plurality of frequency changers coupled in cascade in said channel each having a conversion gain greater than unity and being effective to reduce the signal-carrier frequency, and means for deriving from said frequency-multiplying means highfrequency input oscillations for each of said frequency changers, said last-named means includy'ing means for deriving said input oscillations for one of said frequency changers from another of said frequency changers, whereby an intermediate-frequency signal which has a carrier frequency greatly reduced from that of the received signal and which has a substantially greater am- 'plitude than that of the received signal is derived from the last of said frequency changers.

4. A carrier-signal receiver of the superheterodyne type including a signal-translating channel and adapted to receive a signal having a carrier frequency of the order of 240 megacycles comprising, a low-frequency local oscillator for generating local oscillations having a frequency of the order of 14 megacycles, frequency-multiplying means coupled to said oscillator, a plurality of frequency changers coupled in cascade in said channel each having a conversion gain greater than unity and being effective to reduce the signal-carrier frequency, and means for deriving from said frequency-multiplying means input oscillations for each of said frequency changers, said last-mentioned means including selective means for deriving input oscillations for the second of said frequency changers from the first of said frequency changers, whereby an intimate intermediate-frequency signal which has a carrier frequency of the order of 16 megacycles and which has a substantially greater amplitude than that of the received signal is derived from the last of said frequency changers.

5. An ultra-high-frequency carrier-signal receiver of the superheterodyne type including a signal-translating channel and adapted to receive a carrier of frequency F comprising, a lowfrequency local oscillator, frequency-multiplying means coupled to said oscillator, two frequency changers A and B, respectively, coupled in cascade in said channel each having a conversion gain greater than unity and being effective to reduce the signal-carrier frequency, and means for deriving from said frequency-multiplying means high-frequency input oscillations of frequency fA for frequency changer A and highfrequency input oscillations of frequency JB for frequency changer B, said last-named means including selective means for deriving input oscillations for frequency changer B from frequency changer A, the signal-carrier frequency input to frequency changer B being of frequency F-fl-i, whereby an intermediate-frequency signal of carrier frequency F-fA-JB and which has a substantially greater amplitude than that of the amplitude of the received signal is derived from frequency changer B.

6. An ultra-high-frequency carrier signal receiver of the superheterodyne type including a signal-translating channel comprising, means for generating local oscillations of ultra-high frequency, a plurality of frequency changers coupled in cascade in said channel each having a conversion gain greater than unity and being effective to reduce the signal-carrier frequency, and means for applying said ultra-high-frequency oscillations to each of said frequency changers, said last-mentioned means including selective means for deriving input oscillations for the second of said frequency changers from the rst of said frequency changers, whereby an intermediate-frequency signal which has a carrier frequency greatly reduced from that of the received signal and Which has a substantially greater amplitude than that of the received signal is derived from the last of said frequency changers.

ARTI-TUR I. F. SIMPSON. LUIS C. STEN'NING. 

